This invention relates generally, but not exclusively, to a resonant driving system for a fluorescent lamp having at least one end connected to a primary winding of a transformer.
The invention is particularly, but not exclusively, directed to a resonant driving system for a fluorescent lamp having at least one end connected to a primary winding of a transformer. The system includes an inductor inserted between an input section of the system and an internal circuit node, a converter inserted between the internal node and a voltage reference and comprising a first transistor and a second transistor of the complementary type, inserted, in series to each other, between the internal node and the voltage reference, and a control circuit connected to a secondary winding of the transformer and to the converter as well as to the control terminals of the first and second transistors of the converter.
As is well known, fluorescent lamps are generally made with a tube filled with a mercury-based gas mixture at low pressure. The inner side of the tube is covered by phosphorus or other similar fluorescent elements. When a lamp is turned on, its two electrodes starts to warm up and to emit ions that contribute to fully ionize the gas mixture inside the lamp, facilitating the strike of the arc across the two electrodes.
From an electrical point of view, the resistance of the lamp falls from about one mega ohm down to few hundreds ohm as illustrated in FIG. 1 by the I-V characteristic of a typical low pressure fluorescent lamp at the start up of the lamp.
Once the arc is established, the mercury gas emits radiation in the ultraviolet spectrum that excites the phosphorous coating of the inner side of the lamp. At this time the phosphorous starts to fluoresce producing the light in the visible spectrum.
It should be noted that fluorescent lamps usually require about 600V as peak voltage to strike the arc. Once the arc is established about 100V is enough to sustain it.
In order to avoid the so-called cataphoresis effect, the fluorescent lamps are usually supplied with sinusoidal waveforms by means of an inductor, usually indicated as the ballast. The ballast is connected to a DC-to-AC converter. A capacitive filter is also included to further remove any DC components of the relevant waveforms.
From the I-V characteristic shown in FIG. 1 it is also evident that the voltage across a fluorescent lamp is never equal to the main voltage. In particular, at the start up such a voltage is much higher than the main voltage, and while in steady state it is quite lower. Consequently, the ballast has the important function of generating the high voltage needed to strike the arc and after to generate the inductive reactance needed to reduce the voltage across the electrodes of the lamp. Furthermore the value of inductance L of the ballast must be chosen so that it does not saturate when the lamp is in its operation voltage condition, even at high temperatures.
However, it should be noted that the I-V characteristic shown in FIG. 1 is valid only at the lamp start up. Moreover, the shown characteristic is valid for any traditional magnetic ballasts working at a main frequency. In particular, the negative slope in the characteristic occurs at every cycle near the current zero crossing and it is due to the gas de-ionization. This causes a visible 50 Hz or 60 Hz flicker effect that can be eliminated by increasing the switching frequency of the lamp about three orders of magnitude. In fact at frequencies higher than 20 kHz there is no time for the de-ionization of the gas, and the I-V characteristic of the lamp is linear, as shown in FIG. 2.
The use of a higher frequency also provides two other important benefits: the power consumption is lowered, typically by about 70% due to the no deionization of the gas, with consequent longer life time of the fluorescent lamp;
a smaller inductor in series with the fluorescent lamp can be used, with consequent reduction in the weight of the fluorescent-lamp system.
A simplified block schematic diagram of a system 1 comprising a fluorescent lamp connected to a ballast according to a voltage fed topology is shown in FIG. 3. In particular, the system 1 has a first IN1 and a second input terminals IN2, connected to a main voltage supply and to a filtering block 2. The filtering block 2 in turn comprises a bridge circuit connected to the main supply and a filter, in turn connected to a conversion block 3.
The conversion block 3 substantially comprises a DC to AC converter and is connected to a tube 4 of the fluorescent lamp. In particular, the tube 4 has a first end connected to the conversion block 3 by means of a series of an inductor L (the ballast) and a first capacitor C1 and a second end directly connected to the conversion block 3.
Finally, the tube 4 has a second capacitor C2 connected in parallel to its ends.
According to the topology of the system 1 for fluorescent lamps shown in FIG. 3, the tube 4 is fed by generating an over voltage across the second capacitor C2 through the circuit formed by the series of the inductor L, the first capacitor C1 and the second capacitor C2 itself. At the start up, the system 1 is an open circuit and, by using a second capacitor C2 which is much smaller than the first capacitor C1, such a second capacitor C2 imposes the resonant frequency of the system 1. In such a way, the generated over voltage is high enough to ionize almost instantaneously the gas in the tube 4 of the fluorescent lamp. Once the fluorescent lamp is on, the second capacitor C2 will be short circuited by the fluorescent lamp itself and the natural frequency of the system 1 will be mainly determined by the first capacitor C1. So, the working frequency of the system 1 will be higher than this natural frequency and determined by the DC-AC converter 3.
From all the above, it is evident that core of the system 1 is the DC-to-AC converter 3. In order to clarify the operation of such a DC-to-AC converter 3 reference will be made to FIG. 4, which shows the system 1 of FIG. 3 in a greater detail.
It should be noted that the first capacitor C1 shown in FIG. 3 has been substituted by the two capacitors C5 and C6 (with identical capacitance) to better balance the system 1 as a whole.
In particular, the filtering block 2 of the system 1 comprises a filter 5 connected between the first IN1 and second input terminals IN2 as well as to a diode bridge circuit 6 in turn connected to the conversion block 3.
Moreover, the conversion block 3 has a first IN3 and a second input terminal IN4 and comprises a first resistor R and a third capacitor C3 connected in series between such input terminals IN3 and IN4. The interconnecting node between the first resistor R and the third capacitor C3 is connected to the control terminals of a first Q1 and a second transistor Q2, respectively by means of a series of a diode D0 and a second resistor R0 and by means of a diac D.
In the example shown in FIG. 4, the transistors Q1 and Q2 are bipolar transistors and their control terminals are the base terminals. In particular, the transistors Q1 and Q2 are both of the NPN type, the emitter terminal of the first transistor Q1 and the collector terminal of the second transistor Q2 being connected to an internal node X.
More particularly, the series of the diode D0 and the second resistor R0 is connected to the control terminal of the first transistor Q1 through a first winding of a transformer T and a third resistor R3, while the diac D is connected to the control terminal of the second transistor Q2 through a fourth resistor R4 and to a voltage reference, for instance a ground GND, through a second winding of the transformer T.
The first transistor Q1 is connected to the first input terminal IN3 of the conversion block 3 and to the internal node X by means of a fifth resistor R5. Moreover, a fourth capacitor C4 is connected between the conduction terminals of the first transistor Q1.
The second transistor Q2 is also connected to the internal node X and to the ground GND by means of a sixth resistor R6.
The conversion block 3 also comprises a third winding of the transformer T inserted, in series with an inductor L1, between the internal node X and one end of the tube 4 of the fluorescent lamp.
Finally, the system 1 comprises an electrolithic capacitor C7 connected in parallel to the filtering block 2 in order to provide a stable DC power supply, having a reduced ripple.
At the start up, the first resistor R, the third capacitor C3 and the diac D supply a first current value to the control terminal of the second transistor Q2.
After the start up, the diode D0 inhibits the diac D and the system as a whole is maintained in oscillation by the feedback path to the control terminal of the second transistor Q2 via the transformer T.
In fact, as soon as the second transistor Q2 is on, the current flowing in the primary winding of transformer T (that is a ferrite ring with a saturable core) starts to increase until its core is fully saturated. At this point, the feedback path to the control terminal of the second transistor Q2 is substantially removed and, after its storage time, the second transistor is switched off.
In the mean time, the feedback path realized by the transformer T acts to switch on the first transistor Q1.
It should be emphasized that, once the value of the capacitor C5, and consequently C6, is fixed (according to the power of the fluorescent lamp and the range of frequencies to be utilized), the parameters to be taken into consideration in order to correctly fix the operating frequency of the system 1 are:
The maximum flux density in the core of the transformer T.
The storage time of the transistors Q1 and Q2.
The value of the resistors R5 and R6 connected to the transistors Q1 and Q2 (in correspondence of the emitter terminals in the shown case of bipolar transistors).
It should be also noted that practical considerations impose that the spread of storage time of the used transistors Q1 and Q2 must be tight and known. Also the variability of the magnetic flux in the core of the transformer T must be taken into consideration to correctly dimension the system 1 for the worst case conditions.
FIG. 5 shows, as an example, the steady state waveforms related to a 25 W, 50 Hz-220V mains compact fluorescent lamp driven with a system 1 according to FIG. 4.
Moreover, FIG. 6 illustrates waveforms related to a switch-off condition of the same example lamp discussed in conjunction with FIG. 5.
So, even if effective to drive a compact fluorescent lamp, the known described system 1 shows a main drawback due to the fact that the design of the circuit topology, particularly in connection with the ballast L, depends on the variability of the core saturation in the transformer T as well as on the spread of storage time of the transistors Q1 and Q2.
Also known is a system to supply a fluorescent lamp with a current-resonant-base driving circuit as schematically shown in FIG. 7.
In particular, the resonant driving system 10 is connected to a fluorescent lamp, here represented by its tube 11. A first capacitor C10 is connected in parallel to the tube 11.
The tube 11 of the fluorescent lamp has a first end connected to a first internal node X1 between a second C11 and a third capacitor C12 a well as a second end connected to a primary winding of a transformer TR.
The resonant driving system 10 has a first 11 and a second input terminal 12 connected to a filter 12, in turn connected to a diode bridge circuit 13. The diode bridge circuit 13 is connected to a first inductor L10, which is the ballast of the fluorescent lamp.
The resonant driving system 10 also comprises a DC-to-AC converter 14 inserted between a second internal node X2 (between the first inductor L10 and the second capacitor C11) and a voltage reference, for instance a ground GND.
Finally, the resonant driving system 10 comprises a fourth electrolithic capacitor C14 inserted in parallel to the diode bridge circuit 13 and connected to the first inductor L10 and to the ground GND.
The filter 12, the diode bridge circuit 13 and the fourth capacitor C14 can be seen as an input section of the resonant driving system 10.
The converter 14 comprises a first or high side transistor QH and a second or low side transistor QL of the complementary type, inserted, in series to each other, between the second internal node X2 and ground GND.
In particular, the high side transistor QH is a bipolar transistor of the NPN type, while the low side transistor QL is a bipolar transistor of the PNP type.
The transistors QH and QL have their control or base terminals connected to each other in a third internal node X3. Moreover, the high side transistor QH has its control terminal connected to the second internal node X2 by means of a first resistor RH, while the low side transistor QL has its control terminal connected to the ground GND by means of a second resistor RL.
A fourth internal node X4, defined as the interconnection node between the transistors QH and QL, is connected to one end of the tube 11 of the fluorescent lamp through the primary winding of the transformer TR.
Also shown in FIG. 7 are respective diodes DH and DL connected between the emitter and collector terminals of the bipolar transistors QH and QL.
The current resonant driving system 10 is based on the control circuit 15 inserted between a secondary winding of the transformer TR and the control terminals of the transistors QH and QL. The secondary winding of the transformer TR is also connected to the fourth internal node X4 of the converter 14.
In particular, the control circuit 15 comprises a series of an inductor L15 and a first capacitor CA as well as a second capacitor CB inserted between the third X3 and the fourth internal node X4, also indicated as nodes A and B in FIG. 7. It should be noted that the series of L15 and CA can be seen as a current generator of a driving current I(t) between the nodes A and B. More particularly, the current flowing in the control terminals of the transistors QH and QL plus the currents flowing through the second capacitor CB is equal to such a driving current l(t).
Moreover, as is the common practice in real applications, a snubber circuit 16 is inserted at the output of the DC-AC converter 14 in order to minimize the switching losses of the resonant driving system 10. The snubber circuit 16 comprises a parallel configuration of a capacitor C16 and a resistor R16 and is inserted between the fourth internal node X4 and the ground GND.
It should be noted that the capacitor CB provides a delay time needed for the insertion of the snubber capacitor C16. In fact, the capacitor CB is charged to a voltage equal that of the base-emitter junction of each transistors QH and QL, typically of 1V.
The above-described known system substantially shows a common emitter half bridge topology, thanks to the adoption of the complementary pair of bipolar transistors QH and QL. In fact, the resonant driving system 10 has only one common driving source, in particular the control circuit 15.
It should be noted that the feedback to the transistors has been ensured through a modification of the first inductor L10 or ballast by using just one secondary winding of the transformer TR that accomplishes the task of supplying the voltages needed for the base terminals of the transistors QH and QL with respect to the third internal node X3.
So, the transformer TR need not to saturate at a current level required by the lamp itself. The working frequency is in particular achieved by the LC circuit that results in series with the base terminals of each transistor with a resonant effect.
The current resonant driving system 10 for fluorescent lamps overcomes the problem concerned with the variability of the saturation in the transformer. However, this known solution shows a main drawback due to the fact that the transistors QH and QL have their bases connected together and thus the reverse bias during the off-state is not high enough just 0.8V), especially for the PNP bipolar transistor. Moreover, the transistors QH and QL are driven by a common current resonant circuit that cannot be fine tuned for the specifications of each transistor. The delay time between turn-off of QH and turn-on of QL (and vice versa) is very short and it decreases when the temperature increases (due to the lower Vbe).
An embodiment of this invention provides for a system for driving a fluorescent lamp having at least one end connected to a primary winding of a transformer, the system comprising a pair of transistors of the complementary type and a pair of capacitors connected in series with the control terminals of the transistors in order to allow different driving for the transistors themselves as well as for a control circuit of the voltage resonant type. Such a system overcomes the problems besetting the systems of FIGS. 3, 4, and 7, namely the variability of the saturation in the transformer, the spread of storage time of the transistors, and the driving and sizing of the transistors.